The following are questions posed by participants who attended Suhas’s webinar presentation on “A Scientific Approach to Processing for Injection Molding” on April 13, 2007.

Larry S.Q. Typically how long should it take to establish a robust process from start to finish on a new or existing tool?A. If the mold is built correctly, a typical mold qualification and process establishment should take between three to four hours. This will include setting the mold and melt temperatures and waiting for them to stabilize. If there are issues with the mold, for example, if you get shorts and flash at the same time, then the mold trial must be stopped, and the mold needs to be fixed. In this case, we can still save the viscosity data and continue where we stopped at a later point.

Q. How much of a delta in screw recovery time is acceptable?A. The variation should not be more than 5% of the screw recovery time. For example, if the screw recovery time is about 10 seconds, you should be consistent between 9.5 and 10.5 seconds. For larger shots, you should try and maintain it to about +/- 1 second.

Jerry Z.Q. What is the difference on appearance between crystallized part and uncrystallized part out of the same material?A. Unfortunately you cannot make out by the appearance of the part, if the process for the molded part was optimized or not. In case of mold temperatures, there could be visible differences as follows. Parts molded with a colder mold may appear to be less glossy or shiny, especially for glass filled materials. In some non filled materials there could be a difference in the opacity of the part. For parts molded with a cold mold, the molecules do not have a chance to come together and therefore let the wavelength of light can pass through, making these more transparent.

An annealing test can help in such cases. Measure and record the critical dimensions. Find on the part, two dimensions such that one is in the direction of the flow and the other parallel to it. (These dimensions do not have to be on the part print.) Now, place the part in an oven that is set the temperature close to the maximum recommended mold temperature for the material. After about couple hours, take the part out and let it cool, and re-measure all the dimensions you took before.

The part should not be warped. If it is warped, you have released the stresses and this shows that you had inbuilt stresses (not necessarily because of mold temperature.)

The dimensional difference should not be large. Large would be close to about 50- 75% of the manufacturers published values. If it is then it is obvious that the part has not shrunk as much as it should in the molding process.
Caution: Placing the part in the oven is very tricky. If you place the part on a highly conductive surface such as a metal tray, that particular surface of the part could see additional changes and give you a false reading. The best way to do this is to hang the part inside the oven in such a way that it is not touching the surfaces.

KlenQ. Just to review, you do not recommend a window study for semicrystalline resins because of the narrower melt range?A. You will not need to go to the extremes of the melt temperatures, but you should still do a pressure study.

RoehlQ. Why do nylon or PBT materials drip even with proper drying?A. If the temperature at the nozzle is more than what is needed, then you get drooling. If you reduce it, they will freeze, especially in materials such as nylons. In such cases, where you either drool or freeze off, the nozzle design is very important. There are special Nylon nozzles you can use to prevent this. You can also use the ‘Eliminator’ nozzle from Spirex.

Q. How to determine post mold shrinkageA. Consider you want to perform the study on Dimension A on a particular part. Measure this dimension as soon as the part comes out of the mold and then measure it at 5 mins, 15 mins, 30 mins, 1 hour, 2 , 4, 8, 12, 24, 48 hours till you get that dimension to be constant. When the dimension does not change with time the part has stopped shrinking. Plot this graph out and that will give you the post mold shrinkage for that dimension.

Q. Do you how do you determine the cooling channel size and volume of water needed in a mold?A. You should always have turbulent flow because that will assure you the most efficient heat transfer.

JeremyQ. I believe the slide showing water absorption has too many bonds for Carbon (maximum of four bonds not five bonds) and the weak bond should occur between Oxygen and Hydrogen (not between oxygen and oxygen).A. The slide was only a representation of how water is attracted to the polymer. You are right as far as the number of available bonds for carbon.

EliQ. We have nylon pats with 10% glass that are very brittle after being in the field for 5 years. Is there a way to tell if the nylon was molded/dried poorly, or whether it degraded with time.A. Unfortunately, there is no way to tell after the fact if the nylon was dried properly or not. Degradation can be caused because of various reasons, one of them being hydrolysis from the water present in the plastic.

Q. Are the viscosity curve generation instructions on your website valid for only a velocity controlled press?A. Yes.

Q. For a family with parts of different weights is the pack and hold calculation process the same?A. Since the process is the same, you will be applying the same pressure to all the parts. So whatever pressure you apply to the one part, the same will be applied to the other part. However if the gate is frozen off on one part and not on the other, then the holding pressure will be applied only to the one with the open gate.

About the Author

Suhas Kulkarni FIMMTECH

Suhas Kulkarni is the President of FIMMTECH, a consulting firm that specializes in services related to injection molding. He earned his Masters in Plastics Engineering from the University of Massachusetts, Lowell and a Bachelors in Polymer Engineering from the University of Poona, India. He has 18 years of experience as a process engineer.His main area of expertise is Scientific Processing for Injection Molding. Based on his experience, he has developed a custom software called Nautilus, that aids the complete process development routine to production release.He also teaches a plastics and molding course at the University of California, San Diego and is a contract faculty at the University of Massachusetts at Lowell.The company website is www.fimmtech.com and Suhas can be reached at 760-525-9053.Check out Suhas’s Free Global Resource for Injection Mold Processing at Injection Molding Online

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